As reported recently car would stop running after a few minutes, would re-starting after a short period, always getting voltage to coil, replacing coil, points and cap made no difference.
Latest, car run for two minutes then stopped, no voltage at coil. Replaced the wire from the joint at fuse box to ballast resistor and changed ignition relay when i was down in that area.
Car started first time run till up to temperature. Can only think that this wire had a break or part break and was heating up when running causing a break in voltage to ballast resistor. Only problem now is car is running fast and cant get rev's down below 1200, i have probably caused other issues mucking around trying to fix the original problem.

Car would die, briefly, when driving along - usually in busy traffic - so suddenly that it more or less had to be electrical. This only happened very occasionally.

The short version; I ended up running a supply to the coil directly from the battery through a switch inside the car. When the fault happened I flicked the switch and it went away. Took about a year but eventually traced it to? - you guessed it - the ignition relay.

If you still have a 30 or 40 year old relay in the ignition circuit, change it for decent new one.